cortex-m55: Workaround armasm options bug
cortex-m55: Workaround armasm options bug

We use armclang with `-masm=auto` to auto-select which assembler to use
based on the syntax of the file. Cortex-M55 isn't supported by armasm,
but we don't yet have GCC-syntax asm files for ARM compiler
(https://github.com/ARM-software/CMSIS_5/blob/1dd090bd1c6d196da96241420a40090bf09c0b13/CMSIS/RTOS2/RTX/Source/ARM/irq_armv8mml.s).

    $ armclang --target=arm-arm-none-eabi -mcpu=cortex-m55 -mfpu=none -masm=auto -c cmsis/CMSIS_5/CMSIS/RTOS2/RTX/Source/TOOLCHAIN_ARM/TARGET_M33/irq_armv8mml.S
    armclang: error: armasm does not support CPU 'cortex-m55'

In the mean time, we can build C and C++ files using the
`-mcpu=cortex-m55` option, and for armasm, cancel out that choice of CPU
with a known-supported CPU type, Cortex-R7, and provide the legacy
assembler-specific option `-Wa,armasm,--cpu=cortex-m55`.

After these changes, this works:

    $ armclang --target=arm-arm-none-eabi -mcpu=cortex-m55 -mcpu=cortex-r7 -Wa,--cpu=cortex-m55 -mfpu=none -masm=auto -c cmsis/CMSIS_5/CMSIS/RTOS2/RTX/Source/TOOLCHAIN_ARM/TARGET_M33/irq_armv8mml.S

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